better error that help find the root cause of it.

theano/compile/pfunc.py

@@ -188,9 +188,22 @@ def pfunc(params, outputs=None, mode=None, updates=[], givens=[],
 
         update_val = store_into.filter_update(update_val) # typically this might be a cast()
         if update_val.type != store_into.type:
-            raise TypeError('an update must have the same type as the original shared variable(dest, dest.type, update_val, update_val.type', 
+            #the number of clients don't seam to be defined
+            def get_first_node(node, dtype):
+                if node is None: return None
+                if any([o.dtype!=dtype for o in node.outputs]):
+                    for i in node.inputs:
+                        n = get_first_node(i.owner, dtype)
+                        if n is not None:
+                            return n
+                    return node#no parent generated a different type
+                else: return None
+
+            raise TypeError('an update must have the same type as the original shared variable(dest, dest.type, update_val, update_val.type)', 
                     (store_into, store_into.type,
-                        update_val, update_val.type))
+                        update_val, update_val.type),
+                            'Here is the first node we found that generated a type that is not the same as the output wanted.',
+                            get_first_node(update_val.owner, store_into.dtype))
         update_d[store_into] = update_val
         update_expr.append((store_into, update_val))